1. Field of the Invention
The present invention relates to a forward link device of a multicarrier communication system, and in particular, to a forward link device with improved transmission properties of multicarrier information.
2. Description of the Related Art
With the continuing progress of communication technology, the number of subscribers to communication services is increasing considerably. In addition, many methods have been proposed for satisfying the subscribers' demands for increased service quality. One of the proposed methods is to improve a forward link structure.
A 3rd generation multicarrier CDMA (Code Division Multiple Access) system, which is proposed in TIA/EIA TR45.5, includes an improved forward link of a fundamental channel. FIG. 1 illustrates an improved forward link structure of the multicarrier CDMA system.
Referring to FIG. 1, a convolutional encoder and puncturing part (or a channel encoder) 10 encodes input data into symbols. Here, the input data has a variable bit rate. A symbol repetition part 20 repeats the coded data (i.e., symbols) output from the channel encoder 10 to cause the symbols of different rates to have the same symbol rate. An interleaving part 30 interleaves the symbols output from the symbol repetition part 20. For the interleaving part 30, a block interleaver may be used.
A long code generator 91 generates the same long codes as those used in mobile stations. Different long codes are allocated, as identification codes, to the respective subscribers. A decimator 92 decimates the long codes to match the number of the long codes to the number of symbols output from the interleaver part 30. A mixer 93 mixes the output of interleaver 30 with the output of decimator 92.
A demultiplexer 40 demultiplexes in sequence the coded data output from the mixer 93 to multiple carriers A, B and C. First to third binary-to-4-level converters 51–53, respectively, convert binary data output from the demultiplexer 40 to 4-level data. That is, the level converters 51–53 convert 2-level data to 4-level data. First to third Walsh coders 61–63, respectively, orthogonally spread the data output from the first to third binary-to-4-level converters 51–53 with Walsh codes of length 256 for example, respectively. First to third modulators 71–73, respectively, modulate outputs of the first to third Walsh coders 61–63, respectively. For the modulators 71–73, QPSK (Quadrature Phase Shift Keying) spreaders may be used. First to third attenuators 81–83 control gains of the modulation signals output from the first to third modulators 71–73 according to attenuation control signals GA-GC, respectively. Here, the first to third attenuators 81–83 output the multiple carriers A, B and C being different from one another.
In the multicarrier forward link of FIG. 1, the convolutional encoder and channel encoder 10 having a 1/3 coding rate encodes the input data into three convolutionally encoded bits per input data bit, and the coded data (i.e., FEC codes or symbols) are distributed to the three carriers A, B and C after passing through the symbol repetition part 20 and the interleaver part 30. That is, the multicarrier CDMA forward link of FIG. 1 encodes and interleaves the input data, and then transmits the coded data separately through the three carriers after demultiplexing.
FIG. 2 illustrates a detailed structure of the channel encoder 10 (convolutional encoder and puncturing part), the symbol repetition part 20 and the interleaver part 30. In FIG. 2. the input data has a variable rate: data of a first rate is composed of 172 bits (full rate), data of a second rate is composed of 80 bits (1/2 rate), data of a third rate is composed of 40 bits (1/4 rate) and data of a fourth rate is composed of 16 bits (1/8 rate).
Referring to FIG. 2, first to fourth CRC generators 111–114 add corresponding CRC data bits to the input data, respectively. Specifically, the first CRC generator 111 adds 12-bit CRC data to the 172-bit input data of full rate, the second CRC generator 112 adds 8-bit CRC data to the 80-bit input data of 1/2 rate, the third CRC generator 113 adds 6-bit CRC data to the 40-bit input data of 1/4 rate, and the fourth CRC generator 114 adds 6-bit CRC data to the 16-bit input data of 1/8 rate. First to fourth tail bit generators 121–124 add 8 tail bits to the data output from the first to fourth CRC generators 111–114, respectively. As a result, the first tail bit generator 121 outputs 192 bits, the second tail bit generator 122 outputs 96 bits, the third tail bit generator 123 outputs 54 bits, and the fourth tail bit generator 124 outputs 30 bits.
First to fourth encoders 11–14 encode data output from the first to fourth tail bit generators 121–124, respectively. For the first to fourth encoders 11–14, a convolutional encoder having a constraint length (K) 9 and a coding rate (R) 1/3 may be used. In such a case, the first encoder 11 encodes the 192-bit data output from the first tail bit generator 121 into 576 symbols at full rate. The second encoder 12 encodes the 96-bit data output from the second tail bit generator 122 into 288 symbols at 1/2 rate. The third encoder 13 encodes the 54-bit data output frown the third tail bit generator 123 into 162 symbols at 1/4 rate. The fourth encoder 14 encodes the 30-bit data output from the fourth tail bit generator 124 into 90 symbols at 1/8 rate.
To match rates of the symbols output from the second to fourth encoders 12–14 to the rate of the symbols output from the first encoder 11, second to fourth repeaters 22–24 repeat the symbols output from the second to fourth encoders 12–14 predetermined times, respectively, and third and fourth symbol deletion elements 27 and 28 delete the extra symbols exceeding 576 symbols of the full rate. That is, the third and fourth repeaters 23 and 24 repeat input symbols to output the symbols of approximate full rate. The symbol deletion elements 27 and 28 then delete the extra symbols when the number of the repeated symbols exceeds the number of the full rate symbols, so as to adjust the number of the symbols to the same number of the full rate symbols. Specifically, since the second encoder 12 outputs 288 symbols which is half of 576 symbols output from the first encoder 11, the second repeater 22 repeats the input symbols two times to match the number of the symbols to 576. Further, since the third encoder 13 outputs 162 symbols which is about a quarter of the 576 symbols output from the first encoder 11, the third repeater 23 repeats the input symbols four times to adjust the number of the symbols to 648. Here, since 648 symbols are greater in number than 576 full rate symbols, the third deletion element 27 deletes every ninth symbol to adjust the number of the symbols to the full rate symbol number 576. In addition, since the fourth encoder 14 outputs 90 symbols which is about an eighth of 576 symbols output from the first encoder 11, the fourth repeater 24 repeats the input symbols eight times to adjust the number of the symbols to 720. Here, since 720 symbols are greater in number than 576 full rate symbols, the fourth deletion element 28 deletes every fifth symbol to match the number of the symbols to the full rate symbol number 576.
First to fourth interleavers 31–34 interleave the full rate symbols output from the first encoder 11, the second repeater 22, the third symbol deletion element 27 and the fourth symbol deletion element 28, respectively.
As stated above, the symbol repetition is performed when the number of the input symbols is not equal to the number of the full rate symbols. Here, the carriers A, B and C each have a frequency band of 1.2288 MHz (hereinafter referred to as 1.25 MHz for short) which is equal to the frequency bands of three IS-95 channels. The total frequency band for the three carriers A, B and C is 3.6864 MHz which is approximately 5 MHz. This is equal to the total frequency band for the three IS-95 channels.
The forward error correction (FEC) is used to maintain a bit error rate (BER) at the signal receiver side as low as possible with respect to the channels having a low signal-to-noise ratio (SNR) by providing a coding gain to the channels. The forward link of the multicarrier CDMA system may use an overlay scheme which shares the same frequency bands with the existing IS-95 forward link. The overlay method, however, has the following problems.
One of the proposed overlay methods is to overlay three forward link carriers of the multicarrier system on three 1.25 MHz frequency bands used in the IS-95 CDMA system. FIG. 3 illustrates transmission power levels of the base station by the frequency band, for the IS-95 CDMA system and the 3rd generation multicarrier CDMA system. In this overlay scheme, since the frequency bands of the multicarrier system is overlaid on the existing IS-95 frequency bands, the two systems share the transmission power (including the channel capacity) of the base station in the same frequency bands. In the case when the two systems share the transmission power, after the transmission power of the base station is first allocated to the IS-95 channels, the maximum power which can be allocated to the carriers of the multicarrier CDMA system is determined. Here, the maximum power cannot exceed a specific power level, because the transmission power of the base station has limitations. Furthermore, transmitting data to too many subscribers increases interference, causing an increase in noise. FIG. 3 illustrates the state where the transmission powers can be properly allocated to the IS-95 system and the multicarrier system in the respective 1.25 MHz frequency bands. That is to say, FIG. 3 illustrates the state where the transmission powers of the IS-95 system and the multicarrier system are properly distributed.
However, the transmission powers of the IS-95 channels vary according to the change in the number of the subscribers in service and the change in the voice activity of the subscribers. Thus, in the multicarrier system, the transmission powers allocatable to the respective carriers are variable. FIGS. 4 and 5 show that the transmission powers allocatable to the carriers of the multicarrier system are decreased, when the transmission powers allocated to the IS-95 system increase due to an increase in number of the IS-95 subscribers in service. As a result, if one or more carriers of the multiple carriers are not provided with enough transmission power, the respective carriers have significantly different SNR in the mobile station, and a signal received from the carrier with the low SNR will have the higher BER. That is, under the circumstance that there is a large number of the IS-95 subscribers in service and the voice activity of the subscribers is high, the BER of the signal transmitted through the carrier is increased, which results in deterioration of the system performance. In particular, the IS-95 subscribers are more significantly interfered with, rather than in the independent IS-95 environment. The problems which may be caused by the overlay scheme causes the capacity deterioration of the future multicarrier system and an increase in interferences with the IS-95 subscribers.
In the multicarrier system, the three carriers have independent transmission powers, which cause different performance in each channel. FIGS. 4 and 5 illustrate such an example. From the viewpoint of the performance, FIG. 4 shows a performance similar to that of the base station using a 1/2 rate encoder, and FIG. 5 shows a performance which could be more poor than the performance of the base station not using encoding.
As described above, in the multicarrier system using three carriers extending over a 5 MHz frequency band, when 1/3 rate FEC is used, the encoded data (i.e., syboms) are allocated to the carriers by one symbol, for the case of the full rate fundamental channel, so that each carrier transmits different encoded data. In such a case, failure to obtain one or two bits out of the three encoded data at the mobile station due to the carrier degradation causes the deterioration of performance.